Class 4.1: Flammable Solids or Substances.

Desensitized explosives that when dry are explosives of Class 1 and are specifically authorized by name or have been assigned a shipping name and hazard class by the Associate Administrator.

Self-reactive materials,which are thermally unstable and that can undergo a strongly exothermic decomposition even without participation of air.

Readily combustible solids that can cause a fire through friction and show a burning rate faster than 2.2 mm (0.087 inches) per second, or metal powders that can be ignited and react over the whole length of a sample in 10 minutes or less.